It is commonly known that wind turbines can be placed in areas, where the climatic conditions may cause icing on the wind turbine blades and on the rest of the wind turbine. The risk of ice formation is particularly high at low air temperatures and high humidity or precipitation.
During operation of the wind turbine, ie while the rotor rotates, icing occurs primarily on the blades in the leading edge areas of the blades. This icing may be critical in that the leading edge of the blades greatly affects the aerodynamic properties of the blades, for which reason an ice layer thereon significantly reduces the effect of the wind turbine. Icing on one or more of the blades may further cause an imbalance in the rotor and additional load on the mechanical parts of the turbine.
In order to prevent or at least reduce the problems with icing, either ice removal (also known as de-icing) or ice prevention (also known as anti-icing) is used today. At de-icing a layer of ice is allowed to form on the leading edge of the blade during turbine operation, said layer subsequently being removed at suitable intervals. At anti-icing ice is continuously prevented from forming on the blade, preferably by continuously heating the blade to a temperature above freezing point such that icing thereon does not occur.
De-icing can be performed mechanically, eg at the leading edge of the blade by means of inflatable rubber bellows or thermally by means of electric heating elements embedded in the surface of the blade and feeding hot air to the interior of the blade (known from DE 20 014 238 U1) or by means of microwave energy (known from WO 98/01340).
Anti-icing is primarily performed thermally by heating the entire blade. If only the leading edge is heated as at de-icing, the water produced at the melting of ice flows down towards the trailing edge of the blade and subsequently freezes to ice. At anti-icing it is thus necessary to heat the entire blade.
At present, at weak or no wind, the rotor is stopped or the wind turbine is allowed to idle, whereby the turbine is disconnected from the supply grid, and the rotor thus rotates slowly or stands still depending on the wind speed. In both cases, however, a fairly thick layer of ice forms on the blades, the tower and the nacelle in climatic conditions, where a risk of icing is present.
Before a wind turbine can be restarted and is able to operate efficiently, the ice has to be removed from the blades, which can be effected by heating the blades. The heating may for instance be effected by the methods described above in relation to anti-icing. The ice formed is thus loosened and drops to the ground prior to the start of the turbine.
However, the above described methods of preventing/removing ice from wind turbine blades are not optimum.
Furthermore, when a wind turbine stands still under certain climatic conditions, where there is a risk of icing, all of the turbine components, including the oil in the gear box and in the turbine's various hydraulic components, are chilled. In the stopped state of the turbine, no movement is present in the gear box oil or in the hydraulic oil. Furthermore, the load from the nacelle and the turbine blades rest on the same bearing balls or rollers in various lubrication-free bearings for a long time. Thus, starting the turbine from its stopped state causes more wear and tear than continuous operation of the turbine.